1. Field of the Invention
The present invention relates to an aberration correction system and method of aberration correction used in a scanning transmission electron microscope and, more particularly, to an aberration correction system and method of aberration correction using autocorrelation functions.
2. Description of Related Art
It is very important to correct aberrations produced by the electron optical system of a scanning transmission electron microscope (STEM) in enhancing the resolution of the microscope. Various methods are available for the aberration correction. Among them, a method of aberration correction using an image is disclosed in JP-A-2002-75262. This method is now described.
First, an image A is obtained under a well focused condition. An image B is obtained under an underfocused condition. An image C is obtained under an overfocused condition. The images A, B, and C are Fourier-transformed to give rise to spatial frequency distributions A′, B′, and C′, respectively. Then, the spatial frequency distributions B′ and C′ are divided by the spatial frequency distribution A′. The quotients are inverse-Fourier transformed.
Information about spatial frequencies regarding an observed specimen contained in the image is removed by processing the image in this way. Only aberration information about the electron beam (or the probe) is extracted. Accordingly, aberrations are corrected by manipulating the aberration corrector or deflector based on the aberration information.
In the STEM imaging mode, it is impossible to perform aberration correction by the prior-art method of aberration correction under the condition where an STEM image is being acquired using a Ronchigram or the like. Especially, when high-resolution imaging for imaging atoms or two-dimensional mapping using energy-dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS) is done, it takes a long time to acquire data in many cases. During this time interval, defocus and two-fold astigmatism vary. In this case, the prior-art method of aberration correction that involves interruption of observation cannot be employed. Therefore, it is inevitable that the acquired data will deteriorate in spatial resolution.